all of my strawberries have fruited and are now bare. when is the best time to cut them back

Leave them, watch for the runners to root then move your new plants to another bed. You will then have two beds of strawberries. The oldest bed will tire itself out in about three years.

i have runners growing in small pots. the mother strawberry plants are a mass of leaves

I'd leave them for two reasons:

1. Unless the runners have established themselves, it's the parent plant that is providing all/most of their nutrients.

2. Even though your plants have finished fruiting, they are still growing new roots. I'd prune them back in late autumn.

Do you prune strawberries? I just remove the dead leaves as and when. Plants need leaves, why remove them?

I agree with Maccabee, just let them grow on and give them a tidy up in autumn by removing old and damaged leaves. As for 3 years - while they may be at their most productive while still 'young' they will continue to produce good quantities of fruit for a decade or more. Unless you are desperate to break records or are showing in competitions I wouldn't bother replacing them so early.

I wish I hadn't replaced mine last Autumn. I worked out that they must have been about 7 or 8 years old and the quality of the fruit had deteriorated a bit (though not the quantity). I propagated the runners and planted a new bed, but I've only had a handful of strawberries, the children were disappointed and I can't make any of my own jam! Anyway, I only ever tidied them up each Autumn before that, not "pruned" them.

In my fruit bible, - an RHS book, Fruit by Harry Baker, (ISBN 0 85533 193 3) it recommends cutting off the old leaves and unwanted runners with shears 3" above the crowns in August. By that time, the plants will we looking pretty raggedy anyway and if you use straw, it is a good time to clear the bed and leaving it any later may result in damaging fresh new growth and reducing yeild the next year. Harry Baker recommends burning the straw and leaves.